A communication network is a set of geographically distributed nodes and the links between these nodes for data transmission. The communication between two nodes may be realized by way of intermediate nodes. This saves resources of the communication network and improves the resource efficiency. Present communication networks operate in various types, including SDH/SONET networks and IP networks. Different nodes in a communication network communicate with each other by exchanging data frames or packets, which are defined by special protocols like TCP/IP. The protocol herein means a set of rules defining inter-node interaction, similar to TCP/IP.
When a communication network becomes huge, its management and maintenance will be difficult. Usually to facilitate management, the network is divided into multiple routing domains or autonomous systems (ASs). Usually, in the network inside an AS, traditional intra-domain routers that execute intra-domain routing protocols are coupled together and managed by common powers. For the purpose of improving route retractility, an AS is usually divided into multiple areas. Generally, a domain is a set of any network elements within the scope of a common address management or path computation responsibility. An instance of domain, therefore, may be an area, an AS or multiple ASs. For easy description, herein routing domains, ASs and areas are all referred to as domains. The specific meaning of a domain herein depends on its context. When nodes are added for data exchange, inter-domain routers that run inter-domain routing protocols interconnect the nodes in different domains. Such inter-domain routers are also known as border routers.
An instance of inter-domain routing protocol is the Border Gateway Protocol (BGP) version 4 defined in IETF RFC1771. BGP implements inter-domain routing by exchanging routes and reachable information between adjacent inter-domain routers in the system. BGP usually adopts a reliable transmission protocol like TCP to establish connections and sessions.
An instance of intra-domain routing protocol, or Interior Gateway Protocol (IGP), is the Open Shortest Path First (OSPF) protocol defined in IETF RFC2328. OSPF is based on link state technology and therefore is also a link state routing protocol. The link state routing protocol defines the mode of exchanging and handling intra-domain routing information and network topology information. In OSPF, this information is exchanged through Link State Advisement (LSA).
The emergence of Multi-Protocol Label Switching (MPLS) technology meets the new requirements for data network development. For instance, it provides guaranteed available bandwidth and fast recovery MPLS allows establishing end-to-end tunnels in an IP/MPLS network where there are Label Switched Routers (LSRs). Such a tunnel is generally known as a Label Switch Path (LSP). LSP establishment relates to the computation of a path of a LSR in the network, which is called route computation. MPLS is also introduced to the optical transport network and brings the development of the Automatically Switched Optical Network (ASON). Unlike the traditional optical transport network that provides network connection service through manual or semi-automatic configuration, the ASON provides network connection service through automatic establishment of the control plane. The ASON may be divided into a transport plane that bears network services, a management plane that implements management functions, and a control plane that runs the control protocol.
The control plane of ASON uses Generalized Multi-Protocol Label Switching, which extends MPLS to include the Link Management Protocol (LMP), routing protocol and signaling protocol. LMP obtains the connection types the link supports and the number of resources through message exchange based on discovery of neighboring relations. Such information is known as Traffic Engineering (TE) information and a link that contains TE information is a TE link. Inside a domain, the local TE information is advertised to other nodes in the domain through a routing protocol such as OSPF-TE. Based on this information, when the network management system or a user requests the network to establish a network connection, the ingress node of this connection can perform path computation to obtain the link sequence of the connection and then, through a signaling protocol like the Resource Reservation Protocol-Traffic Engineering (RSVP-TE), send a request to the nodes on the path for resource allocation and establish a cross connection. In this way, an end-to-end connection is established.
For both JP/MPLS and optical transport networks, the division of domains is a concern. Especially, when a network with TE management capabilities is divided to multiple domains, each node only knows the TE information of the local domain and the reachable information of other domains. This reduces the impact of topology change on new service deployment and congestion recovery. The network extendability is enhanced. As each node knows only the TE information of the local domain and the reachable information of other domains but does not know the complete TE information of other domains, it becomes an issue how to compute a path that meets all the constraints on end-to-end bandwidth, switching capability, route separation, protection, and user policies in the case of multiple domains.
To solve the route computation or path computation (hereinafter uniformly referred to as route computation for easy description) in the case of multiple domains, the Domain-Domain Routing Protocol (DDRP) adopts a hierarchical network model, in which, a lower layer domain is represented by an agent node. The agent node can advertise abstract topologies, inter-domain links, and reachable addresses that represent the domain. Thus a hierarchical network takes shape. When an end-to-end path that crosses multiple domains is computed, the strict route in the domain of the requesting node and the subsequent loose route of the border node are first computed. When signaling flows to the border of an intermediate domain, the strict route in this intermediate domain is computed by means like domain border computation. This continues until the signaling goes to the domain of the destination node. The defect of this solution is that route computation is a serial process. At the head node, only the ingress and egress information of some domains the path passes is available. Because the real TE information in related domains is unavailable, judgment may be made on whether path computation can succeed from the ingress to egress only when signaling flows to the corresponding domain border and triggers domain border computation. Thus, it may frequently happen that signaling goes to the middle way to discover that no route is available or no root satisfies related constraints. As a result, route establishment is rolled back multiple times, and the previously established cross connection has to be removed for re-establishment. In addition, with the DDRP technology, it is hard to compute end-to-end diverse routes (different paths with the same source and destination).
Another technology that solves the route computation in the case of multiple domains is Path Computation Element (PCE). Each PCE stores all TE information of the domain it serves (for ease of description, the TE information here includes network topology information). A node requesting route computation is called a Path Computation Client (PCC). The PCC sends a request that contains route computation parameters to the PCE, and the PCE performs route computation according to the Traffic Engineering Database (TED) it stores and sends the result to the PCC. A PCE may store TE information of one or multiple domains. When a route crossing multiple domains is computed, if the route is beyond the service area of the local PCE, the PCE will use the Path Computation Element Communication Protocol (PCECP) to collaborate with other related PCEs to compute the final route. The PCE technology adopts a flat single-layer model, in which, all PCEs are equally important. When the network is complex or large, it is difficult to manage the PCEs. In addition, as hierarchical abstraction is not applied to the network, inter-domain route computing entirely relies on the exchange of TE information of different domains between different PCEs. When a service passes through many domains, communication between related PCEs will be too frequent and the volume of information exchanged will be huge. This will reduce the efficiency and reliability of route computation.